As an apparatus for forming a thin film, for example, a silicon oxide film (SiO2), on a substrate such as a semiconductor wafer (hereinafter, referred to as a “wafer”) or the like, there is known so-called a mini-batch-type film forming apparatus which performs a film forming process on a plurality of substrates mounted on a rotary table along a circumferential direction. In this film forming apparatus, for example, the rotary table is made of quartz. The substrates are heated by a heater disposed below the rotary table. By rotating the rotary table, the substrates sequentially pass through a raw material gas adsorption region, a reaction gas supply region and a plasma-based modifying region. Thus, thin films are formed on the substrates by so-called an atomic layer deposition (ALD) method and are modified by plasma.
Examples of a gas for generating plasma include an easy-to-ignite argon (Ar) gas. There may be a case where plasma is not ignited due to delicate state fluctuations such as variations in a concentration distribution of the Ar gas, a flow of the Ar gas and an electric power supplied to the semiconductor wafer. If the rotary table is rotated and the film forming process is performed without igniting plasma, it is impossible to perform plasma modification. Thus, the consumed processing time is wasted. In the meantime, the light coming from a heater passes through the rotary table. Therefore, a plasma generation region remains bright even when plasma is not generated in the plasma generation region. For that reason, if a worker is responsible for the confirmation of the plasma ignition, it is difficult for the worker to confirm the ignition or non-ignition of plasma. Thus, the worker feels burdensome.
In addition, there is known a plasma-based substrate etching apparatus which uses a technique of monitoring the emission of plasma and detecting an abnormality in the emission of plasma based on a value corresponding to a difference between a maximum value and a minimum value of color components R, G and B on the basis of a brightness signal corresponding to a predetermined state of plasma. However, such a technique cannot solve the problem addressed in the present disclosure.